1. Field of the Invention
This invention relates to an electric-signal amplifying device, and more particularly, it relates to a semiconductor device capable of amplifying an electric-signal at a very high speed by use of optical transmission.
2. Description of the Prior Art
FIG. 4 shows a conventional electric-signal amplifying device using optical transmission. This device has a multi-layered structure in which an n.sup.+ -GaAs sub-collector layer 22, an n-GaAs collector layer 23, a p-AlGaAs base layer 24, a p-GaAs base layer 25, an n-AlGaAs emitter layer 26, and an n.sup.+ -GaAs sub-emitter layer 27 are formed on a semi-insulating GaAs substrate 21 in this order.
In the production of this device, the above-mentioned layers are successively grown on the semi-insulating GaAs substrate 21, after which mesa etching is performed to remove the layers on part of the p-GaAs base layer 25 and also remove the layers on part of the n.sup.+ -GaAs sub-collector layer 22. On the exposed surfaces of the p-GaAs base layer 25 and n.sup.+ -GaAs sub-collector layer 22, a base electrode 29 and a collector electrode 210 are formed in an annular shape, respectively. An emitter electrode 28 is formed on the n.sup.+ -GaAs sub-emitter layer 27.
FIG. 5 shows energy levels in the layers of the electric-signal amplifying device of FIG. 4 in operation. A power source is connected between the emitter electrode 28 and the collector electrode 210 to apply a voltage V.sub.ce to the base-collector junction in the reverse direction. Another power source is connected between the emitter electrode 28 and the base electrode 29 to apply a voltage V.sub.be to the emitter-base junction in the forward direction. This results in an emitter current, by which electrons are injected from the n-AlGaAs emitter layer 26 into the p-GaAs base layer 25. The injected electrons recombine with holes in the p-GaAs base layer 25, thereby causing recombination radiation. The light emitted from the p-GaAs base layer 25 is absorbed by charges in the emitter electrode 28 and n.sup.+ -GaAs sub-collector layer 22, thereby causing excitation which generates electron-hole pairs. The thus generated electrons form an output collector current Ic.
Therefore, a current gain .beta. of the conventional electric-signal amplifying device is given by the following expression. EQU .beta.=.eta..sub.1 .eta..sub.2 /(1-.eta..sub.1 .eta..sub.2)
where .eta..sub.1 is the quantum efficiency for the radiative recombination at the emitter-base junction and .eta..sub.2 is the quantum efficiency for the light absorption at the base-collector junction.
In the conventional electric-signal amplifying device, an input electric signal is amplified through optical transmission to produce an output electric signal. The light emission in the vicinity of the emitter-base junction is caused by spontaneous recombination through the injection of minority carriers (i.e., electrons) into the p-GaAs base layer 25. However, radiative recombination associated with minority carriers requires a certain period of time. This prohibits a quick response of the device, thereby lowering the operation speed thereof.